Lithography is a technique used for integrated circuit fabrication in which a silicon slice is coated uniformly with a radiation-sensitive film, the resist, and an exposing source (such as light, x-rays, or an electron beam) illuminates selected areas of the surface through an intervening master template for a particular pattern. Although well known for use in the manufacture of semiconductors, lithography has a broad range of industrial applications. Such applications include, for example, flat-panel displays, micro-machines, and disk heads.
The lithographic process allows for a mask or reticle pattern to be transferred via spatially modulated light (the aerial image) to a photoresist film on a substrate. Those segments of the absorbed aerial image, whose energy exceeds a threshold energy of chemical bonds in the photoactive component of the photoresist material, create a latent image in the photoresist. In some photoresist systems, the latent image is formed directly by the photoactive component. In others, called acid-catalyzed photoresists, the photochemical interaction first generates acids which react with other photoresist components during a post-exposure bake to form the latent image. In either case, the latent image marks the volume of photoresist material that either is removed during the development process (in the case of positive photoresist) or remains after development (in the case of negative photoresist) to create a three-dimensional pattern in the photoresist film.
The principal determinant of the photoresist image is the surface on which the exposure energy equals the photoresist threshold energy in the photoresist film. "Exposure" and "focus" are the variables that control the shape of this surface. Set by the illumination time and intensity, exposure determines the average energy of the aerial image per unit area. Local variations in exposure can be caused by variations in substrate reflectivity and topography. Set by the position of the photoresist film relative to the focal plane of the imaging system, focus determines the decrease in modulation relative to the in-focus image. Local variations in focus can be caused by variations in substrate film thickness and topography.
Generally, because of variations in exposure and focus, patterns developed by lithographic processes must be continually monitored or measured to determine if the dimensions of the patterns are within acceptable ranges. The importance of such monitoring increases considerably as the resolution limit, which is usually defined as the minimum feature size that can be resolved, of the lithographic process is approached. The patterns being developed in semiconductor technology are generally in the shape of lines, both straight and with bends, having a length dimension equal to and multiple times the width dimension.
The width dimension, which by definition is the smaller dimension, is on the order of 0.1 micron to greater than 1 micron in many of the advanced semiconductor technology. Because the width dimension is the minimum dimension of the patterns, it is the width dimension that challenges the resolution limits of the lithographic process. In this regard, because width is the minimum and most challenging dimension to develop, it is the width dimension that is conventionally monitored to access performance of the lithographic process and is referred to as the "critical dimension."
There are a variety of dry etch process types, including reactive ion etching (RIE), physical sputtering, and plasma etching. RIE is a conventional method used to pattern a substrate. RIE combines a physical basis (ion) and a chemical basis. In processes that rely predominantly on the physical mechanism of sputtering, the strongly directional nature of the incident energetic ions allows substrate material to be removed in an isotropic manner (i.e., essentially vertical etch profiles are produced). Unfortunately, such material removal mechanisms are also non-selective against masking material and materials underlying the layers being etched. Thus, the selectivity depends largely on sputter yield differences between materials. Because the sputter yields for most materials are within a factor of three of each other, selectivities are typically inadequate. Moreover, because the ejected species are not inherently volatile, redeposition and trenching can occur. Another problem with pattern transfer by physical sputtering involves the redeposition of nonvolatile species on the side walls of the etched feature. Consequently, dry etch processes for pattern transfer based on physical removal mechanisms have not found wide use in very large scale integrated circuit or VLSI (i.e., more than 100,000 devices per chip) fabrication processes.
In contrast, dry processes relying on chemical mechanisms for etching can exhibit high selectivities against both mask and underlying substrate layers. Such purely chemical etching mechanisms typically etch, however, in an isotropic fashion. Although some applications in VLSI fabrication (such as photoresist stripping in oxygen plasmas) use such processes, the problem of undercutting associated with isotropic etching remains.
By adding a physical component to a purely chemical etching mechanism, the shortcomings of both sputter-based and chemical dry etching processes can be overcome. Dry etch processes based on a combination of physical and chemical mechanisms offer the potential of controlled anisotropic etching together with adequate selectivity. Therefore, RIE is a preferred dry etch process.
The step of reactive ion etching a substrate is typically followed by a final critical dimension measurement operation. That operation measures the metrology of the etched substrate to confirm proper etch performance. Conventional etch performance monitoring uses a scanning electron microscope (SEM). SEM metrology has very high resolving power and is capable of resolving features on the order of 0.1 micron.
Unfortunately, the process of measuring the critical dimension of an etch using SEM metrology suffers from several drawbacks. First, a SEM is very expensive, requiring a tremendous capital expenditure. SEM metrology is also very slow in operation, typically requiring five minutes or more to analyze a reactive ion etched substrate. Furthermore, SEM metrology is difficult to automate because it requires a time consuming pattern recognition step.
As a result of these deficiencies, analysis using scanning electron microscopy is typically done on a single substrate per batch of, for instance, twenty four substrates. It is impractical to use SEM metrology to analyze all substrates in a particular batch. Thus, analysis of the performance of a reactive ion etch on a substrate using scanning electron microscopy suffers from several drawbacks.
The deficiencies of the use of a SEM to measure RIE performance show that a need exists for a technique for monitoring RIE performance that is less expensive to implement, faster in operation, and easier to automate than existing processes.